Mississippi Flood

3-10-07 The Need For River Management River Flooding: The Mississippi, 1993 Flooding by rivers is a natural event which, because people often choose to live in flood-risk areas, becomes a hazard. To people living in the Mississippi valley, that their river should flood is as natural as sunshine I Florida or snowfall in the Rockies’. Without human intervention , the Mississippi would flood virtually every year. Indeed, it has been this frequency of flooding which has over many centuries, allowed today’s river to flow for much of its course over a wide, fertile, flat, alluvial floodplain. Usually, of course, the great floods occur in the lower river, in the last 1,600km below Cairo, Illinois. This is where the plain flattens out )the river drops less than 120m from here to its mouth) and where the Ohio and the Tennessee flow into the Mississippi. Of the water that flows past Memphis, only about 38 per cent comes from the MissouriMississippi network. The bulk comes from the Ohio and Tennessee, from the lush Appalachians, rather than the dry mid-West. “We don’t mind too much about eh Missouri”, says Donna Willett, speaking for the US Army Corps of Engineers (who have the responsibility of flood prevention). “It can rain there for weeks, and we wouldn’t mind. We can handle three times the water coming down in those floods. But the Ohio, well, that’s another story. When that starts rising, we start watching…” Where is the river/drainage basin located? What is the frequency of flooding? What is the magnitude of flooding?

What are the natural causes of flooding?

The Mississippi-together with its main tributaries, that Missouri and the Ohio-drains one-third of the USA and a small part of Canada. Left to its own devices, flooding would be an almost annual event with late spring being the peak period. Until recently, major floods occurred every 5-10 years (there were 6 in the 1880’s) and a serious/extreme flood occurred approximately once every 40 years. Usually it results from heavy rainfall (January-May) in the

What are the consequences of flooding?

What attempts can be made to reduce the flood hazard?

How successful have the attempts to reduce flooding been?

Appalachian Mountains, especially if this coincides with snowmelt. Initially, it was to develop the wide, alluvial floodplain. The 1927 flood caused 217 deaths, 700,000 people were evacuated, the river became up to 150km wide (usual width 1km), livestock and crops were lost and services were destroyed. Until the 1927 flood, the main policy was “hold y levees”-by 1993 some levees were 15m high. After 1927, new schemes including building dams and storage reservoirs (six huge dams and 105 on Missouri); afforestation to reduce/delay runoff; creating diversion spillways (e.g. Bonnet Carre floodway diverts floodwater into Lake Pontchartrain and the sea_; cutting through meanders to straighten and shorten the course. In 1883, Mark Twain calimed that “You cannot tame the lawless stream”. By 1973, it appeared that the river had been tamed: there was no further flooding…until 1993. Has human intervention made the danger worse?

Engineering/planning schemes in the Mississippi Basin Prior to the 1993 flood, it was perceived that the flow of the Mississippi had been controlled. This had been achieved through a variety of flood prevention schemes. - Levees had been heightened, in places to over 15m and strengthened. There were almost 3000km of levees along the main river and its tributaries. - By cutting through meanders, the Mississippi had been straightened and shortened for 1750km, it flows in artificial channels. - Large spillways had been built to take excess water during times of flood

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The flow of the major tributaries (Missouri, Ohio and Tennessee) had been controlled by a series of dams.

Why did the Mississippi flood in 1993? The Mid-West was already having a wet year when record-setting spring and summer rains hit. The rain ran off the soggy ground and into rapidly rising rivers. Several parts of the central USA had over 200 per cent more rain than was usual for the time of year. It was the ferocity, location and timing of the flood that took everyone by surprise. Normally, river levels are falling in midsummer, the upper Mississippi was not perceived to be the major flood-risk area, and people believed that flooding in the basin had been controlled. Floodwater at St Louis reach and all time high. Satellite photographs showed the extent of the flooding. After the flood: should rivers run freer? Since the first levee was built on the Mississippi in 1718, engineers have been channelling the river to protect farmland and towns from floodwaters. But have the levees, dams and diversion channels actually aggravated the flooding? There are two schools of thought. One advocates accepting that rivers are part of a complex ecological balance and that flooding should be allowed as a natural event. The other argues for better defences and a more effective control of rivers. Flood of 93 Nearly half of the counties in nine states bordering the upper reaches of the Mississippi and Missouri rivers have been declared federal disaster areas. This is the first step in becoming eleigible for federal aid, including direct graints from Congress, Federal Emergency Management Agency and many other groups. Damage: $10.5 billion Deaths: 45 Evacuated: 74,000 Houses: 45,000 Crops: $6.5 billion Illinois: In the fight against flooding rivers 17 levees were breached, including one that flooded the town of Valmeyer and 70,000 acres of surrounding farmland. One flood-related death was reported IN Alton, the treatment plant was flooded 1st August cutting off water to the towns 33,000 residents. “Our Levee did not breach, but the water came in through the street, the drains, anywhere there was a hole, at such a rate that the pumps couldn’t keep up.” Says Mayor Bob Towse. Statewide property losses may top £365 million including damage to

140 miles of roads and eight bridges. Agricultural damage is estimated at more then $610 million. An estimated 4% of the states cropland900,000 acres-was flooded. In addition 15727 were displaced, 860 businesses closed and nearly 9,000 jobs lost. Missouri: The highest death toll-25-and the greatest property damage-$1.3 billion-of all flooded states were reported here. Statewide, 13 airports have been closed, and 25,000 residents evacuated. Flooding on 1.8 million acres of farmland has caused about $1.7 billion in crop losses. Heroic efforts have apparently saved historic Ste Genevieve which has been battling rising waters since the start of July.